As of August 26, 333 dolphins have washed up dead or dying on beaches from New York to North Carolina, Teri Rowles, a coordinator with NOAA Fisheries marine mammal health and stranding response program, said at a press conference. Virginia seems to be a "hot zone," reporting 174 dolphin strandings as of Monday. (Take a look at the numbers, state by state.)

Cetacean morbillivirus is in the same family as the virus that causes measles in people. But this group of viruses tends not to jump from species to species, said Jerry Saliki, a virologist at the University of Georgia who has been conducting laboratory analyses of samples from the stranded dolphins.

Even so, NOAA officials are warning the public not to approach stranded dolphins as they could have secondary bacterial or fungal infections that could pose a risk to people, especially those with open wounds. (Related:"New Diseases, Toxins Harming Marine Life.")

"Along the Atlantic seaboard, this [outbreak] is extraordinary," Rowles said. The last morbillivirus outbreak in the region occurred from June 1987 to May 1988, and resulted in the deaths of at least 900 bottlenose dolphins. (Related:"U.S. Dolphin Deaths Rise to 300; Cause Still a Mystery.")

Officials are unsure of how long the current outbreak will last. "Typically, outbreaks will last as long as there are susceptible animals," Rowles said.

But if it plays out like the 1987-1988 outbreak, "we're looking at mortality being higher and morbillivirus traveling southwards and continuing until May 2014," she added.

Right now, experts think this current outbreak is probably due to a dip in "herd immunity."

Dolphins that survived the 1987-88 morbillivirus outbreak carry antibodies to the virus which protects them against it. These resistant individuals also help protect new or young dolphins without natural immunity, since these unprotected individuals have less chance of contracting the virus.

But eventually, those virus-resistant dolphins die or leave, and "then the whole population becomes susceptible," said Saliki.

Studies have shown that dolphins younger than 26 years have limited to no immunity to this virus, said Rowles. "So if this virus is introduced, they don't have the initial antibodies to protect them from significant illness."

Rowles added that environmental factors, such as heavy metal pollution and sea surface temperature changes, could also play a role in the current outbreak. But researchers are still gathering data to answer these questions.